We are investigating approaches for utilizing recombinant DNA techniques in developing vaccines for human respiratory syncytial virus (RSV). Vaccinia virus recombinants expressing the RSV F or G glycoprotein were highly immunogenic and effective in inducing resistance to RSV infection in rodents and monkeys but, surprisingly, these recombinants were not protective in chimpanzees. To investigate the participation of individual RSV proteins in host immunity, recombinant vaccinia viruses were constructed which individually expressed seven additional RSV genes, namely the 1C, 1B, N, P, M, 22K M2, and SH genes, and construction of a recombinant for the remaining gene, L, is underway. These recombinants are being evaluated for immunogenicity, protective efficacy, and possible disease potentiation in rodents. To date, the SH and N proteins did not induce significant protective immunity, in contrast to the high levels of protective immunity elicited by the F and G glycoproteins. The 22K M2 protein was found to be the major viral target antigen for RSV-specific murine cytotoxic T lymphocytes, with F and N being secondary targets, but the importance of cytotoxic T cells in immunity or disease enhancement remains to be defined. The use of adenovirus as a viral vector offers several potential advantages for RSV immunoprophylaxis, and previously a type 5 recombinant expressing the RSV F protein was constructed and shown to induce high levels of protective immunity in cotton rats. This work is being extended by constructing parental adenovirus vectors containing various engineered insertion sites which are designed to minimize alteration of adenovirus gene expression and replication. These vectors will be used to construct recombinants expressing the RSV F protein that will be evaluated for (1) replicative capacity, (2) alteration in pathogenesis during immunization, (3) immunogenicity and protective efficacy of the expressed RSV protein, and (4) the possibility that immunization potentiates pulmonary pathology during subsequent infection with RSV.